1. Field of the Invention
The present invention relates to a thin film transistor (TFT) and a manufacturing method for a TFT, and more particularly, to a thin film transistor for a liquid crystal display device such as a liquid crystal projector, an organic EL display device, etc., and to a manufacturing method therefor.
2. Description of the Related Art
In recent years, there has been realized an enhanced high performance and an enhanced high function of liquid crystal display devices (LCD), organic electroluminance (EL) display devices and the like of an active matrix type which is used for display devices of information electronic apparatuses such as personal computers and televisions.
Recently, it has been possible to manufacture liquid crystal, or EL display devices of an active matrix type including a drive circuit on an inexpensive glass substrate using poly-silicon (hereinafter referred to as poly-Si) TFTs.
Since the poly-Si TFTs are low in light sensitivity and are high in mobility as compared with a-Si TFTs, and hence miniaturization of pixel TFT has been possible, the poly-Si TFTs are useful for a liquid crystal light valve for a data projector for which miniaturization and enhanced definition are required under an environment having a high light intensity.
In addition, since the poly-Si TFTs are adapted to cause therethrough a large current as compared with the a-Si TFTs, the poly-Si TFTs are also suitable for drive devices for organic EL display devices.
In general, as shown in FIG. 1, in addition to a thin film structure similar to a general direct viewing type liquid crystal panel including an active layer 701, a gate insulating film 702, a gate line 703, a data line 704, and a pixel electrode 705, a ground light shielding film 706 for protecting a TFT from a reflected light of a projected light from a rear face of a substrate is included for a poly-Si TFT liquid crystal light valve.
On the other hand, the a-Si TFT has a bottom gate structure, and a gate electrode serves as a light shielding film as well. But, in the a-Si TFT, a back channel side on an upper surface of a semiconductor layer becomes a main cause of a leakage current. For example, Japanese publication application H4-367276A and Japanese publication application H4-349637A discloses a technique for a-Si TFT in which a back channel surface is plasma-processed to suppress a leakage current.
In addition, the organic EL display device, as shown in FIG. 2, has a TFT 801 and a light emission layer 802 on the TFT, and a light from the light emission layer is directed towards a TFT substrate side. Thus, a ground light shielding film for protecting a TFT from a reflected light from a rear face of the substrate becomes necessary in some cases depending on a quantity of light from the EL light emission layer.
In general, a film made of metal, including metal compound, which is excellent in light reflection characteristics is used as a light shield film. However, since the metal film is electrically conductive, the light shield film operates a back channel of a TFT to change the TFT property. For suppression of the back channel operation by the light shield layer, a technique for sufficiently increasing a thickness of a base interlayer between a ground light shield film and a TFT active layer, or applying an electric potential to the light shield film is disclosed in a patent literature 3, for example. In accordance with this patent literature 3, a technique is disclosed in which when an electric potential of a light shield film is in a floating state, a thickness of an interlayer film is made equal to or larger than 0.8 μm, or an OFF electric potential of a TFT is applied to the light shield film.
However, if a thickness of a base interlayer film is increased, then a quantity of light becomes large which passes through the base interlayer film to enter an active layer. As a result, a photo leakage current is increased to cause reduction of contrast in a display device. On the other hand, if a thickness of the base interlayer film is decreased, then the TFT property becomes sensitive to an electric potential of the light shielding film. Then, when for an n-channel TFT, a negative voltage (OFF electric potential) is applied to the light shielding film as schematically shown in FIG. 3, a leakage current due to the back channel operation has a tendency to be increased.
On the other hand, when the voltage applied to the light shield film is a positive voltage, the leakage current is reduced due to the back channel operation. However, since a threshold voltage is shifted to the negative side, a current value at an OFF operating point of a TFT is increased.
As mentioned above, in a case where the base interlayer film is thinned and a voltage is applied to the light shield film, it becomes difficult to decrease the leakage current with high stability.
As a result, the OFF operation margin in individual TFTs becomes small, and hence the scattering in leakage current values among the TFT devices becomes large to cause reduction in luminance, reduction in contrast and display nonuniformity in a display device.
The above-mentioned operation is schematically shown in FIG. 4. Here, a gate voltage is an OFF voltage. When a thickness of a base interlayer film is large, a leakage current stably shows a small value against a light shield voltage, but a contribution of a photo leakage current becomes large. On the other hand, when the thickness of the base interlayer film is small, the contribution of the photo leakage current is small, but a leakage current becomes sensitive to a light shield voltage due to the back channel operation.